1. Field of the Invention
The present invention relates to the direct preparation of premicronized low optical density magnetic material from submicron ion exchange resin using hollow fiber ultrafiltration technology.
The present invention further relates to magnetic materials having tunable magnetic properties, and more specifically, the present invention relates to magnetic materials containing both single-domain and multidomain particles. More particularly, the present invention relates to magnetic materials having high initial permeability while maintaining coercivity and remanence in the pigment.
The present invention also relates to a method for making low optical density magnetic fluids containing both single-domain and multidomain particles. More specifically, the invention relates to a method for the preparation of colored magnetic particles and ferrofluids using various colorants, dyes or pigments.
Finally, the invention relates to xerographic magnetic liquid toners, colored xerographic magnetic liquid toners and liquid ink compositions and methods of preparation thereof.
2. Description of Related Art
In standard one and two component xerographic and other magnetic imaging systems, the magnetic pigment used has both a remanence and coercivity that enables the pigment to function in the applied field, Due to the remanence and coercivity properties of the magnetic pigment, prior art materials required high weight or volume loading of the pigment in order to get an initial permeability high enough to make the material useful.
Magnetic pigments having high initial permeability are desirable because they allow for substantially lower pigment loadings which in turn improves the rheological properties of a toner or developer or improves the optical properties of, for example, a single component highlight color or color clean machine subsystem.
Prior Art superparamagnetic (SPM) materials for use as magnetic pigments provide the desired high initial permeability. These materials are not entirely satisfactory as they have no coercivity or remanence which are necessary for certain applications, i.e. any application requiring a memory. Such superparamagnetic materials have no memory in that, they are only magnetic in the presence of a field and have no net magnetism outside the field.
Ferrofluids which contain superparamagnetic materials as described above, are recognized within the prior art for a number of applications, including exclusion seals for computer disc drives, seals for bearings, for pressure and vacuum sealing devices, for heat transfer and damping fluids in audio speaker devices and in inertia damping. Typical prior art superparamagnetic materials such as those described by Wyman in U.S. Pat. No. 4,855,079 are coated particles which are in an organic based carrier material. More specifically, this patent discloses a superparamagnetic material which is formed by the precipitation of the magnetic particles (magnetite). These particles are subsequently coated with an oleic acid surfactant. The coated particles were eventually suspended in an organic dispersing agent. The use of dispersing agents or surfactants is a problem in that the right or enabling surfactant must be found empirically. Furthermore, the surfactant may degrade or cause adverse chemical reactions in the magnetic fluid during its application.
A method of forming dry magnetic particles by precipitation of a magnetic oxide in an ion exchange resin is discussed and exemplified by Ziolo in U.S. Pat. No. 4,474,866, which is incorporated herein by reference. According to the method employed, an ion exchange resin is loaded with a magnetic ion. The resin is then recovered and dried. The loaded resin does not contain single-domain and multidomain crystallites internal and external to the resin bead, respectively. The magnetic polymer resin must then be micronized to form a fine magnetic powder. The micronization step is a time and energy intensive process. The dry magnetic particles formed according to Ziolo, U.S. Pat. No. 4,474,866, like other typical prior art materials, could not be directly suspended in an aqueous medium to form a stable colloid.
None of the heretofor known prior art magnetic materials contain both single-domain and multidomain crystallites. Domain as used herein is described for example in C. P. Bean and J. D. Livingston, J. Appl. Physics 30, 120s (1959) and B. D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Publishing Co., MA, (1972), both of which are incorporated herein by reference. The presence of both single-domain and multidomain crystallites provides the ability to tune the magnetic properties to match the desired use for the material. By varying the amount of single-domain and multidomain crystallites with respect to one another it is possible to provide a material whereby the properties of high initial permeability, remanence and coercivity may be varied relative to one another.